Point to Settings -> Other Branding Settings-> and then click Default Language.

Select the radio button next to the desired language.

Click Change.

This will change the language on the Control Panels as well as any notification mails sent out by the system to your Customers and Sub-Resellers. However, your Customers and Sub-Resellers also have the option to set their language preference for themselves. This would over-ride your specified preference, for that particular Customer or Sub-Reseller.